Many PBX telephone systems have or allow the use of advanced, multibutton, multifeature electronic telephones. In some cases, the PBX system is designed for use only with such electronic telephones. In an effort by the manufacturer to provide the most benefit to the user, many of these electronic telephones are equipped with a handsfree (speakerphone) feature. However, in order to keep the price of the electronic telephone competitive, the design of the handsfree feature is often subject to feature benefit v. feature cost tradeoffs, sometimes resulting in low sensitivity, excessive talk-to-listen switching delay time, false activation by noise, etc. Therefore, a user will often purchase an external speakerphone specifically designed to avoid these disadvantages.
Also, many users have computers and desire to have these computers exchange data with other computers over standard telephone lines. Typtically, the computer is interfaced to the telephone line by a modem (modulator-demodulator).
FIG. 1 shows a typical prior art method of connecting an external device 13 (external speakerphone or modem) to a telephone system comprising a telephone switch 10 and one or more electronic telephones 12.
Telephone switch 10 is connected to one or more telephone company central office trunk lines 8. ports VPS and DNS of switch 10 are connected by a four-conductor cable 11 to one port of a "T"-connector 9. A second port of "T"-connector 9 is connected to the VPT and DNT ports of electronic telephone 12 by four-conductor cable 14. The third port of "T"-connector 9 is connected to the V and C ports of external device 13 by four-conductor cable 15. External device 13 is typically a speakerphone or a modem.
External device 13, when activated, will typically place a short across the terminal of its control (C) port. If a conventional "T"-connector is used for connector 9 then, when external device 13 is activated, conductors 11a and 11d, and 15a and 15d, of cables 11 and 15 will be shorted. Telephone switch 10, in response to this short, will typically disable its VPS and DNS ports until the short is removed. Therefore, a modified "T"-connector is generally used for connector 9. The modified "T"-connector 9 has no connecting pins for conductors 15a and 15d of cable 15, so when the external device 13 shorts conductors 15a and 15d, conductors 11a and 11d will not be shorted and telephone switch 10 will allow its VPS and DNS ports to remain enabled.
However, even if external device 13 is successfully connected to telephone switch 10 and electronic telephone 12 other problems frequently occur.
For example, if external device 13 is a speakerphone and external device 13 is activated then acoustical feedback may occur. It will be appreciated by anyone who has experienced acoustical feedback that the piercing sound generated is extremely unpleasant and quite effective in causing the user to press every button he can find on electronic telephone 12 in a desperate attempt to stop the feedback. Frequently, the connection with the called party is broken during this activity. Even more frequently, the user abandons use of the speakerphone and uses the less convenient regular handset to avoid this problem.
Of course, the acoustical feedback problem is, in reality, resolved rather easily by first pressing the microphone "MUTE" key on electronic telephone 12, if it is so equipped, then placing the handset, preferably speaker down, onto a sound absorbent surface, such as a handkerchief or a coat. However, counting the last step of turning on external device 13, three steps are required to change the communication path from electronic telephone 12 to external device 13. Likewise, three steps are required to change back to electronic telephone 12. This makes it inconvenient to establish calls using external device 13 or to switch the communication path back and forth between electronic telephone 12 and external device 13. Therefore, there is a need for a device which will allow the communications path to be switched from electronic telephone 12 to external device 13 by simply pressing the "on" button on external device 13.
If external device 13 is a modem, a different set of problems occurs. First, the user must typically take the handset off hook and/or turn down the volume on electronic telephone 12 to avoid hearing the "noise" and tones of the modem data transfer signals.
Another problem is that room noise and conversations will be picked up by electronic telephone 12 and placed onto cables 11 and 15 via cable 14. The room noise and conversations will appear as noise on top of the modem data transfer signals, thereby introducing phase and amplitude distortion into the modem data transfer signals and degrading or prohibiting the transfer of data by the modems. Of course, this can be remedied by enabling the microphone mute feature of electronic telephone 12.
Therefore, in order to use the modem (external device 13) the user must first turn down the volume on electronic telephone 12, then mute the microphone on electronic telephone 12, and finally, activate the modem (external device 13). Likewise, three steps are also required to change from use of a modem back to use of electronic telephone 12. Therefore, there is a need for a device which will allow the communications path to be switched from electronic telephone 12 to external device 13 by simply activating external device 13.
It is, of course, a relatively simple matter to switch audio conductors 11b and 11c from electronic telephone 12 to external device 13 if the characteristics of the switch and electronic telephone sets are compatible with standard 2 wire telephone sets. In this type of system, d.c. operating voltage is supplied to each phone set over a tip/ring wire pair which also carries audio information. However, in some PBX telephone systems, the audio conductors 11b and 11c form one leg of the power supply for electronic telephone 12 while the other leg of the power supply is provided on a data pair. In other words, in such a PBX there is no d.c. voltage between the conductors of the audio pair. If audio conductors 11b and 11c are simply switched from electronic telephone 12 to external device 13, electronic telephone 12 will be deprived of operating power and stop communicating with telephone switch 10. In such a case, telephone switch 10 will determine that electronic telephone 12 is malfunctioning, disable its VPS and DNS ports, and drop any connection to the central office trunk line 8, thereby preventing external device 13 from being used.
The present invention was designed for use with this typte of PBX system. One example of such a system currently available as of the filing date of this application is the EZ-1/96, manufactured by Isoetec.
Electronic telephone 12 does not perform dual-tone multiple-frequency (DTMF) dialing. Instead, electronic telephone 12 sends data signals over conductors 11a and 11d to telephone switch 1O. Telephone switch 10, in response to the data signals, generates the DTMF tones and, for the user's convenience, places these tones onto conductors 11b and 11c. If the user has been connected to a central office trunk line 8 then telephone switch 10 also places the DTMF tones onto trunk line 8.
There is therefore a need for a device which, in response to external device 13 being activated, will disable the audio to and from electronic telephone 12 without interrupting power to electronic telephone 12 or causing electronic switch 10 to determine that electronic telephone 12 is malfunctioning.
Furthermore, since external device 13 is generally limited in the functions it can perform (typically, only DTMF dialing), there is a need for a device, as above, which also allows electronic telephone 12 to perform most of its normal functions (on hook/off hook, dialing, do not disturb, speed dialing, last number redial, callback, transfer, conferencing, etc,), even when external device 13 is activated.
In telephone conferences between the inventor and technical personnel at Isoetec, the technical personnel stated that such a device was not possible because of the nature of the telephone switch 10 and the electronic telephone 12. Furthermore, the technical personnel indicated that, because of the manner of providing power, data signals, and audio signals, the only proper method of connecting an external device 13 to telephone switch 10 was by using an Isoetec interface card which caused external device 13 to be treated as a separate station and use another DNS/VPS port pair in telephone switch 10. The price of the interface card and the desire to avoid using another DNS/VPS port pair were some of the factors leading to the present invention.